1. Field of the Invention
This invention relates to a novel process for the formation of a light-sensitive silver halide emulsion precipitated by reaction of solutions of halides and a silver salt in the presence of a protective colloid or peptising agent, the emulsion grains consisting of at least two halides of different atomic numbers, which results in a particularly uniform distribution of the halides both within particular zones or throughout the entire volume of the individual grain and from grain to grain, so yielding advantages in sensitivity, fog, gradation, sensitivity to substances inhibiting or accelerating development. These advantageous properties may be used to produce not only improved photographic products but also photographic products which, for a given range of properties, are distinguished by a reduced application of silver halide and reduced quantities of compounds which influence development, for example DIR couplers, DAR couplers.
2. Description of the Prior Art
In the case of mixed silver halides, the desire to arrange the distribution of the different halides both within the individual grain and from grain to grain as uniformly as possible is known from EP 563 708, as improvements in properties are expected from such an arrangement.
The measures which have hitherto been proposed are complicated, technically costly and the results achieved are still inadequate. They are based on chemical and processing measures or combinations of the two. Examples are:
a) pAg-controlled silver halide precipitation, which may be automated and is thus the simplest, but still does not result in the desired uniformity; PA1 b) the use of silver halide micrate emulsions of a predetermined composition, for example pure silver iodide emulsions, the silver halide of which dissolves during the actual precipitation operation and is reprecipitated together with other silver halides; PA1 c) the use of external mixing chambers; and PA1 d) currently the most recent stage, the use of organic compounds which release iodide ions as a function of pH.
Methods b) to d) require considerable additional complication and appreciably increase silver halide production costs without achieving more than slight improvements.
Micrate emulsions are here taken to be particularly finely precipitated silver halide emulsions with an average diameter of a sphere of an equal volume to the grain of less than 0.1 .mu.m.